GW realised the awesomeness that is the Karaz Ankor could only be contained for so long and, eventually coming to their senses, decided to move them up the queue. Too late to come out ahead of Bretonnia and Tomb Kings of course, but they spared themselves from an even bigger Grudge. For now.
In all seriousness, it could just be that the Summer Empire vs Orcs & Goblins rumour was false, or that Dwarfs are coming sooner. Or this just managed to slip through the cracks somehow and it's been sat in a warehouse forever long.
When you think about it, if GW are planning on re-releasing armies in pairs (like Tomb Kings vs Bretonnia) it actually makes the Summer rumour more suspect. There's not enough armies on the list for them all to be 1v1 pairings, and the Empire being in the middle of a civil war - as well as their recent surge in popularity due to Total War - makes them the most fitting faction to drop on their lonesome.
Dwarfs vs Orcs & Goblins
Wood Elves vs Beastmen
High Elves vs Warriors of Chaos
Empire (vs Empire)